A metrology tool for measuring structures on wafers and on masks used in the manufacture thereof is described in detail in the manuscript of the presentation “Pattern Placement Metrology for Mask Making” by Dr. Carola Biasing, issued for the conference Semicon, Education Program in Geneva on Mar. 31, 1998. The description therein relates to the fundamentals of a metrology tool. The structures of wafers or on masks used for exposure only permit very narrow tolerances. For checking these structures therefore a very high precision of measurement (currently in the nanometer to subnanometer region) is required. A method and a metrology tool for determining the position of such structures is known from the German patent application DE 100 47 211 A1. For details on the mentioned determination of positions explicit reference to this document is made.
The German patent DE 1 04 B4 discloses a method for determining the position of edge sections in a mask coordinate system. In a metrology tool the mask is placed on a measurement stage which is displaceable orthogonally to the optical axis of an imaging measurement system, the displacement of the measurement stage being interferometrically measured. The position of a mask coordinate system with respect to the coordinate system of the metrology tool is determined based on at least two structures on the mask. The relative distance of one of at least the first and the second outer edge to the at least two structures is determined. The metrology tool determines the actual coordinates of the at least two structures to the respective outer edges, which must not exceed a certain deviation from a reference value.
Furthermore a metrology tool is known from a plurality of patent applications, like for instance from DE 19858428, from DE 10106699, or from DE 102004023739. In all prior art documents mentioned a metrology tool is disclosed with which structures on a substrate can be measured. Therein the substrate is placed on a measuring stage movable in X-coordinate direction and in Y-coordinate direction. The metrology tool therein is set up in such a way that the positions of the structures, or of the edges of the structures, respectively, are determined with an objective. For determining the position of the structures, or of their edges, respectively, it is necessary for the position of the measuring stage to be determined with at least one interferometer. Finally the position of the edge with respect to a coordinate system of the metrology tool is determined.
The masks to be measured in a metrology tool are intended for the exposure of wafers in an exposure apparatus. For this purpose they are inserted into a mask frame in the exposure apparatus. In order for the insertion to occur in a reproducible manner, the mask makes contact with three points for lateral guidance; these contact points are also known as banking points. For the manufacture of wafers the position of the structures on the mask with respect to these contact points is important, in particular as within the exposure apparatus the mask can only be moved to a certain extent in order to correct deviations of the positions of the structures on the mask from an ideal position. In order to determine this relative position, in the mask coordinate system both the positions of the structures on the mask and the positions of the predefined sections of the outer edges, which in the exposure apparatus make contact with the contact points, are determined.
In a prior art metrology tool a reflection means on the measuring stage or on the mask frame is used to determine the position of the outer edge from the measured brightness distribution of the reflected light via image processing. Visible light (wavelength region approximately 400 nm to 750 nm) is used for this position determination. For measuring the coordinates of structures UV light (365 nm, 266 nm, 248 nm, 193 nm, . . . ) is used. The prior art reflection means is a metal mirror also reflecting the UV light. This reflection perturbs the measurement of the coordinates of structures and reduces the precision of the measurement, a fact which is unacceptable in view of modern and future chip generations.